Insights into the dependence of galaxy properties on the environment with explainable machine learning models

Galaxies reside within dark matter haloes, but their properties are influenced not only by their halo properties but also by the surrounding environment. We construct an interpretable neural network framework to characterize the surrounding environment of galaxies and investigate the extent to which their properties are affected by neighbouring galaxies in IllustrisTNG300 data (z=0). Our models predict galaxy properties (stellar mass and star formation rate) given dark matter subhalo properties of both host subhalo and of surrounding galaxies, which serve as an explainable, flexible galaxy–halo connection model. We find that prediction accuracy peaks when incorporating only the nearest neighbouring galaxy for stellar mass prediction, while star formation rate prediction benefits from information from up to the third-nearest neighbour. We determine that environmental influence follows a clear hierarchical pattern, with the nearest neighbour providing the dominant contribution that diminishes rapidly with additional neighbours. We confirm that central and satellite galaxies, as well as different galaxy categories based on mass and star-forming activity, exhibit distinct environmental dependencies. Environmental dependence for low-mass galaxies (log (M_∗/M_⊙) < 10) shows 35–50 percent environmental contribution compared to just 8–30 percent for massive centrals, while satellite galaxies experience consistently stronger environmental effects than centrals across all populations. Furthermore, we find that the most significant attribute from neighbouring subhaloes for predicting target galaxy properties is its distance to the nearest neighbouring galaxy. These quantitative results offer guidance for constructing more sophisticated empirical and semi-analytic models of galaxy formation that explicitly include environmental dependence as a function of galaxy type and mass.